Add experimental `completebasecase` protocol

[codecov[bot]]

Codecov Report

Merging #509 (2601be1) into master (3541502) will decrease coverage by 0.88%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   93.45%   92.57%   -0.89%     
==========================================
  Files          32       32              
  Lines        2217     2223       +6     
==========================================
- Hits         2072     2058      -14     
- Misses        145      165      +20     

Flag	Coverage Δ
Pkg.test	89.65% <100.00%> (+0.03%)	:arrow_up:
Run.test	92.39% <100.00%> (-0.89%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/combinators.jl	96.61% <100.00%> (+0.31%)	:arrow_up:
src/core.jl	89.76% <100.00%> (-0.80%)	:arrow_down:
src/processes.jl	94.19% <100.00%> (ø)
src/reduce.jl	72.56% <0.00%> (-9.89%)	:arrow_down:
src/basics.jl	88.88% <0.00%> (-5.56%)	:arrow_down:
src/partitionby.jl	90.90% <0.00%> (-3.90%)	:arrow_down:
src/progress.jl	91.58% <0.00%> (-0.94%)	:arrow_down:
src/library.jl	95.81% <0.00%> (+0.25%)	:arrow_up:

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